The idea behind this is that counting higher numbers takes much time and therefore it normally isn't done quite often. If you are only moving a single discussion which is one of lets say 1.3k in a category, this is nothing which can be noticed.

If you are starting a forum it shows every time

But if you have only a few discussions I would say it is better for building a community if you have less categories. Trust on tags if you think you need something to group them.